In electrical power cable connector applications, a desirable feature is the flexibility of the connector for ease of installation. This is particularly important for the so-called pre-molded “slip-on” connectors which are not factory pre-stretched and where a tight fit is required to prevent moisture ingress to the cable connection (which can lead to electrical failure).
These connectors generally comprise an inner semiconductive layer, a thicker insulation layer and an outer semiconductive layer which covers the entire body of the part. The desired level of conductivity of the semiconductive layer is measured by the volume resistivity of the material which for this type of application is less than (<) 1000 ohms per centimeter (ohm-cm) and preferably <500 ohm-cm. Preferably, the volume resistivity is stable under cable operating conditions (generally 0 to 95° C.).
For a typical part made of an ethylene/propylene/diene monomer (EPDM) based material, the amount of conductive carbon black needed to achieve the required volume resistivity is generally greater than (>) 30 weight percent (wt %). This is known as the percolation threshold. Since carbon black acts as a reinforcing agent, this level of conductive filler can significantly reduce the flexibility of the material to such a point that the semiconductive layer is much stiffer compared to the insulation layer. In other words, the flexibility of the finished molded part is severely compromised because of the higher stiffness of the outer and inner semiconductive layers.
Formulation approaches using plasticizing oils and waxes to improve flexibility are known in the literature, but are generally limited in scope and teaching. Thus, a need exists for technology to improve the flexibility of the semiconductive layer in order to improve the overall part flexibility, yet maintain the desired volume resistivity of the cable at its expected operating conditions. One approach is the use non-polyolefin based resins, e.g., silicone rubber for the semiconductive compound. Such technology exists and is in use, for example, in the so-called cold shrink connectors. However, the cost of these materials is significantly higher compared to that of polyolefin-based compounds. Moreover, the tear strength of silicone rubber is generally lower compared to the tear strength of polyolefin rubber materials.